It has been reported by, for example, O. Nakamura et al. in Materials Research Bulletin 17, 231 (1982) that a heteropoly-acid (solid) such as dodecatungstophosphate (H.sub.3 PW.sub.12 O.sub.40.nH.sub.2 O) is a proton-conductive solid electrolyte. However, no heteropolyacids containing carbon as a heteroatom have been known as yet.